Final answer:
The national ensign is flown at the stern when a ship is underway. This location is chosen to display the ship's nationality and is a standard practice in maritime tradition.
Step-by-step explanation:
When underway, the national ensign is typically flown at the stern of the vessel. This tradition is observed across various navies and commercial vessels. The national ensign, often referred to as the national flag, is a significant symbol of a ship’s nationality and is flown from the stern to denote its origin to others when at sea.
The bow of the ship is the front part, while the stern is the rear part. The mast is the tall vertical structure on the ship, and the helm is where the ship is steered from. According to nautical custom, the national flag should be flown at the stern, which is the correct answer, Option A, as the question dictates when the ship is underway, which means it is moving or navigating through water.